The invention relates to an above-ground fluid storage tank, and in particular to a fluid storage tank, with a valve and spill containment system.
When gas is extracted from the ground, it is often extracted along with volumes of water mixed with particulate solids such as sand and carbon. The gas is separated from the mixture and the water mixed with solids is diverted to a storage tank for temporary storage in the field.
Similarly, when crude oil is pumped out of the ground, often it is pumped along with a mixture of sand and water. This mixture is pumped directly to above-ground oil storage tanks for temporary storage in the field. Because the proportion of water to oil varies from oil well to oil well, these tanks must have valves at different levels on the tank from which oil or water is drawn off.
Although these fluid storage tanks may vary in size from 100 barrel capacity to over 100,000 barrel capacity, a common size tank is in the 750 to 1000 barrel range. Such a tank is normally cylindrical, has a radius of less than 8 feet and stands approximately 25 to 32 feet high.
Within these tanks, the stored fluid settles and the solids sink to the bottom. With oil storage tanks, a series of valves and taps are provided on the exterior of the tank to draw off or sample the fluid at different heights within the tank. A common arrangement is to provide a loading spout and valve at about the 12 foot mark, a series of sample taps at the three foot, six foot, nine foot and 12 foot marks and an outlet valve and nozzle at about the three foot mark, alternatively a system of risers may be used to extract fluid from different levels of the tank. With water storage tanks used in gas production, it is common to simply have a single drainage valve and tap positioned towards the base of the tank. Normally, fluid is drawn out of the tanks through the outlet valve by a vacuum truck which applies a vacuum to the tank through a hose.
These external valves and taps, and especially the outlet valve and nozzle at the three foot mark, are subject to the elements. Abrasive dirt and dust may cause premature wear or seizure of the valve. It may corrode. In colder weather, the valve may freeze-up due to moisture despite the fact these tanks are usually heated from within by a burner tube. When the valve does freeze-up either it is forced open or steamer trucks must be called in to thaw the frozen valve and to allow the oil or water in the tank to be withdrawn. Often the valve cracks or breaks when it is forced open. Replacing the valve is an expensive and time-consuming operation as the tank must be emptied. Calling a steamer truck is also expensive because of the down-time while the vacuum truck stands by idly waiting for the services of the steamer truck.
Above-ground fluid storage tanks also create risks of environmental damage. There is always spillage from the three foot outlet valve or the loading spout whenever a hose is connected or disconnected. Also, these tanks are known to overflow if they are not emptied on a regular basis. The overflow occurs through vents and thief hatches normally provided at the top of the tank and the fluid runs down the exterior of the tank onto the ground.
One cumbersome prior art solution to the freeze-up problem is to wrap these valves in insulation, to prevent freezing. However, this insulation may come loose and further, deteriorates quickly as it becomes saturated with oil and salt water. Also, insulating the valve does not always ensure the valve does not freeze in very cold weather.
Another prior art solution is to build a cabinet around the valve on the exterior of the tank. This cabinet does not have a floor; it is open to the ground. However, this solution also does not prevent freezing of the valve in very cold weather and provides only limited containment of spills and leakage from the valve in that the ground is still exposed.
In U.S. Pat. No. 5,960,826, a spill containment chamber which is enclosed within the tank is disclosed. This chamber took the form of a cabinet which was welded to the interior surface of the tank. An opening into the cabinet was cut into the tank wall to provide access to the valves contained within the cabinet. This solution is somewhat awkward when retrofitting the internal chamber into existing tanks. This solution is also not well adapted to double-walled tanks, which are desired or required in many situations. In addition, the insertion of this type of chamber into tanks with a coated interior often results in a breach of the integrity of the coat necessitating a recoating process which is both time consuming and expensive.
There is therefore a need in the industry for a fluid storage tank with a valve and spill containment system, where the valves are kept in a warm and dry environment, protected from the elements, but still readily accessible, and, further, where spillage from the valves is contained. It would be preferred if the containment system allowed for convenient retrofit into existing tanks as well as new tank manufacture, provided that during retrofit insertion the integrity of any pre-existing coating should be maintained.